London is the capital city of England and the United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its founding by the Romans, who called it Londinium. London's ancient core, the City of London, largely retains its square-mile mediaeval boundaries. Since at least the 19th century, the name London has also referred to the metropolis developed around this core. The bulk of this conurbation forms the London region and the Greater London administrative area, governed by the elected Mayor of London and the London Assembly.

London is a leading global city, with strengths in the arts, commerce, education, entertainment, fashion, finance, healthcare, media, professional services, research and development, tourism and transport all contributing to its prominence. It is the world's leading financial centre alongside New York City and has the fifth-largest city GDP in the world (and the largest in Europe). London has been described as a world cultural capital. It has the third most international visitors in the world and London Heathrow is the world's busiest airport by number of international passengers. London's 43 universities form the largest concentration of higher education in Europe. In 2012 London will become the first city to host the modern Summer Olympic Games three times.

London has a diverse range of peoples and cultures, and more than 300 languages are spoken within its boundaries. In July 2010 Greater London had an official population of 7,825,200, making it the most populous municipality in the European Union, and accounting for 12.5% of the UK population. The Greater London Urban Area is the second-largest in the EU with a population of 8,278,251, while London's metropolitan area is the largest in the EU with an estimated total population of between 12 million and 14 million. London had the largest population of any city in the world from around 1831 to 1925.

London contains four World Heritage Sites: the Tower of London; Kew Gardens; the site comprising the Palace of Westminster, Westminster Abbey, and St Margaret's Church; and the historic settlement of Greenwich (in which the Royal Observatory marks the Prime Meridian (0° longitude) and GMT). Other famous landmarks include Buckingham Palace, the London Eye, Piccadilly Circus, St Paul's Cathedral, Tower Bridge, Trafalgar Square and Wembley Stadium. London is home to numerous museums, galleries, libraries, sporting events and other cultural institutions, including the British Museum, National Gallery, Tate Modern, British Library, Wimbledon and 40 theatres. The London Underground is the oldest underground railway network in the world and the second-most extensive (after the Shanghai Metro).